Thermoplastic polymeric materials are used extensively in automotive vehicles and for other purposes. They are light and relatively easy to fashion into complex parts, and are therefore preferred instead of metals in many instances. However a problem with some metal alloys and some polymers is salt stress (induced) corrosion cracking (SSCC), where a part under stress undergoes accelerated corrosion when under stress and in contact with inorganic salts. This often results in cracking and premature failure of the part.
US Patent Publication 2010/0233402 entitled “Salt Resistant Polyamide Compositions” discloses certain semi-aromatic copolyamides that exhibit improved chemical resistance especially to metal halides and salts compared to corresponding aliphatic homopolyamides. In these copolyamides, at least 15 mole percent of the repeat units are derived from monomers that comprise an aromatic structure. Thus, semi-aromatic copolyamides 612/6T comprising 20 to 30 mole percent 6T units exhibit improved salt resistance than corresponding homopolyamide PA 612.
The presence of two or more types of repeat units in a copolyamide, however, has a negative consequence. These copolyamides have reduced degree of crystallinity. As a result they exhibit inferior high temperature properties compared to the corresponding homopolyamides. These properties include such mechanical properties as stiffness, strength and creep resistance at high temperature that are important in many of their end-uses. As the aromatic repeat unit content increases to approach 50 mole percent, the polymer becomes increasingly amorphous, and correspondingly exhibits greater loss in high temperature properties.
When molar aromatic repeat unit content exceeds 55 percent as in case of polyphthalamides, the copolyamide is able to develop crystallinity and exhibit improved high temperature properties. However, these copolyamides have very high melting points often exceeding 300° C. They are less desirable for applications requiring extrusion processing such as hose and tubes, cable covering and filaments. For these applications, it is desirable to have polyamides that exhibit melting points below about 290° C.
The salt resistance and high temperature mechanical properties are thus two conflicting aspects influenced by the aromatic repeat unit content of the semi-aromatic copolyamides. It is desirable to develop semi-aromatic copolyamide formulations that simultaneously exhibit good salt resistance and high temperature properties, and are processable at temperature below 300° C.